Process for the purification of pyrite cinders from nonferrous metals, from arsenic and from sulfur

ABSTRACT

A process for the purification of pyrite and pyrrhotite cinders from nonferrous metals, from arsenic and from sulfur. The process is characteristic in that the reduction of the hematite to magnetite is carried out in a fluid bed by direct injection of a hydrocarbon fuel and air in deficiency, at 850*-950* C. and with contact times from 15 to 45 minutes, up to a degree of reduction equal to from 10-90 percent, and in the presence of small quantities of HC1. The chlorination and reoxidation of the thus produced hot cinders is carried out in a fluid bed reactor with air and a chlorinating agent, at temperatures of from 650*1,000* C., preferably at 850*-950* C. with contact time from 20 to 150 minutes, and that the gases leaving the reactor contain less than 0.5 percent by volume of free oxygen.

United States Patent Colombo et al.

[54] PROCESS FOR THE PURIFICATION OF PYRITE CINDERS FROM NONFERROUSMETALS, FROM ARSENIC AND FROM SULFUR [72] Inventors: Umberto Colombo;Giuseppe Sironi; Bruno Viviani; Ariano Colombini, all of Novara, Italy[73] Assignee: Montecatini Edison S.p.A., Milan, Italy [22] Filed: July24, 1969 [21] Appl. No.: 844,600

[30] Foreign Application Priority Data July 26, 1968 Italy 19452568 [52]US. Cl ..75/9 [51] Int. Cl. C22b 1/08, C22b H10 [58] Field of Search..75/6, 9, 7

[56] References Cited UNITED STATES PATENTS 2,848,314 8/1958 Johannsenet a1. ..75/9 3,160,496 12/1964 Vaccari et al ..75/9 X 3,235,328 2/1966Lerner et al. ..75/113 X Mar. 14, 1972 FOREIGN PATENTS OR APPLICATIONS554,003 3/1958 Canada ..75/9 593,959 3/1960 Canada ..75/9 802,037 9/1958Great Britain ..75/9 818,747 8/1959 Great Britain ..75/7

Primary Examiner-Allen B. Curtis Attorney-Curt M. Avery, Arthur E.Wilfond, Herbert L. Lerner and Daniel J. Tick [57] ABSTRACT A processfor the purification of pyrite and pyrrhotite cinders from nonferrousmetals, from arsenic and from sulfur. The process is characteristic inthat the reduction of the hematite to magnetite is carried out in afluid bed by direct injection of a hydrocarbon fuel and air indeficiency, at 850950 C. and with contact times from 15 to 45 minutes,up to a degree of reduction equal to from 10-90 percent, and in thepresence of small quantities of I'ICl. The chlorination and reoxidationof the thus produced hot cinders is carried out in a fluid bed reactorwith air and a chlorinating agent, at temperatures of from 650-l ,000C., preferably at 850-950 C. with contact time from 20 to 150 minutes,and that the gases leaving the reactor contain less than 0.5 percent byvolume of free oxygen.

8 Claims, No Drawings PROCESS FOR THE PURIFICATION OF PYRITE CINDERSFROM NONFERROUS METALS, FROM ARSENIC AND FROM SULFUR As is known, pyritecinders and pyrrhotite cinders, in order to be used in iron metallurgy,must have a very high-iron content and must be almost free of nonferrousmetals such as Cu, Zn, Pb, as well as free from As and S. The maximumpermissible limits for these impurities have steadily been dropping andat present, a good commercial product should contain not more than0.03-0.05 percent of Cu, Zn, Pb and not more than 0.01-0.03 percent ofAsand S.

The nonferrous metals are eliminated by transforming them into chloridesor soluble sulfates and then by removing the salts with an acidleaching, by converting the metals into chlorides with Cl HCl, CaCl etc.and then by removing said chlorides from the cinders by voltilization athigh temperature. The elimination of the As is either carried out in thecourse of roasting the pyrite or in the coarse of the variouspurification stages such as the reduction to magnetite, the leaching andthe pelletization at high temperature. The removal of sulfur from thecinders is carried out partly during the various stages of the abovementioned purification.

IN general, the cinders have, at the end of these processes, a still toohigh content in sulfur, except those transformed into pellets hardenedat a temperature above l,l50 C.

US. Pat. application Ser. No. 649,279, filed June 27, 1967 of Colombo etal., and now U.S. Pat. No. 3,499,754 describes a process for thepurification of pyrite cinders from nonferrous metals (such as Cu, Zn,Pb, Au, Ag, Ni, Co, Cd and Mn).

The process of this Application comprises:

a. Preheating at temperatures between 600 and 850 C. and partial ortotal reduction (20-100 percent) of the hematite to magnetite byinjecting a carbonaceous fuel and air into the fluid bed reactor. Theair is insufficient for the total combustion.

b. Chlorination and oxidation of the reduced cinders at temperaturesbetween 650 and 950 C., in a fluid bed reactor. The chlorinating gasmixture, consisting of air and chlorine, flows in countercurrent to thecinders. The quantity of chlorine, fed to the reactor, corresponds tothe stoichiometric quantity for the formation of the nonferrouschlorides, with an excess of 5-20 percent.

0. Scrubbing the metal chlorides vapors to obtain an aqueous solution,from which the metals are recovered, by conventional hydrometallurgicalprocesses.

The cinders freed from the nonferrous metals, but still containingsulfur, are directly conveyed to the high-temperature pelletizing stageif their contents in iron is sufiiciently high, otherwise they are firstsubjected to magnetic concentration (beneficiation) after a previousmagnetic reduction. The sulfur is volatilized as S0 during the hardeningof the pellets at a high temperature.

According to an improvement of this process, described in US. Pat.application Ser. No. 840,058, filed on July 8, 1969 by Colombini et al.,and based on Italian Pat. application No. 18688 A/68 of July 8, 1968,the reduction is conducted at higher temperatures (850-950 C.) and atsufficiently long contact times (30-90 minutes) in order to decomposethe ferrous arsenate. The subsequent chlorination is carried out inoxidizing atmosphere by keeping in the exit gases an oxygenconcentration above 3 percent by volume. In this way the arsenic, stillpresent after the chlorination stage, is in the form of solublearsenate, removable by acid leaching of the purified cinders. However,in this case also, the residual sulfur is fully removed only during thehardening of the pellets at high temperature.

The transformation into pellets of the cinders containing sulfur,requires very high temperatures (from l250 to 1350 C which temperaturesare near the incipient softening point of the material. For this reason,it is necessary to adopt pelletizing systems, such as grade pelletizing,which will ensure a sufficiently homogeneous distribution of thetemperature throughout the bed for facilitating a continuous operationof the plant. Such systems are, however, economically convenient onlyfor high capacities (of at least 1,000 t/d, (ton day) owing to the highcost of the plant and to the low-scale factor, and to the operatingcosts. This explains the little use of the pelletizing technique in thetreatment of pyrite cinders.

This shows how important it is to obtain cinders free of sulfur in orderto be able to adopt cheap pelletizing systems also for plants oflow-output capacities, for instance in systems operating at lowtemperature (less than 600 C).

We have now found that the operating conditions of the reduction andchlorination phases may be chosen in such a way as to obtain, at the endof the two phases, ashes purified, not only from nonferrous metals, butalso from arsenic and sulfur. The final cinders, purified under suchconditions, show a content of Cu, Pb, Zn of less than 0.05 percent, andfor content in As and S of less than 0.02 percent. The final cindersthus obtained do not require any supplementary treatment, except for theenriching in Fe (in case that the contents in Fe is still low), and inparticular they do not require acid leaching for the removal of theresidual arsenic, nor they require any pelletizing at high temperaturefor the removal of the residual sulfur. These cinders may thus finddifferent uses in the metallurgical industry, for instance they may beconverted into pellets by means of cheaper low-temperature hardeningprocess or they may be directly subjected to reduction in order toobtain a prereduced material (at the limit: iron sponge), with a lowcontent in sulfur.

According to this invention, the heating and the reduction of thecinders is carried out in a fluid bed, by direct injection into thereactor of a hydrocarbon fuel, air and a small quantity of HCl Chlorine,C1 also may be used, which is transformed into HCl under the reactionconditions. The operation at high temperature (850 950 C.) and in thepresence of HCl brings about a high reductionv and desulfurization rateand facilitates the voltilization of the arsenic, prevailingly as As Oand AsCl The quantity of HCl may vary from a minimum of 40 percent ofthe quantity necessary for volatilizing all the As present as AcCl to amaximum of percent of the quantity required for volatilizing arsenic andlead as chlorides. The degree of conversion of the hematite intomagnetite is between 10 and 90 percent and depends essentially on thethermal balance of the subsequent chlorination phase. The contact timeis comprised between 15 and 45 minutes.

Still according to this invention, the chlorination and the reoxidationof the cinders thus reduced, are carried out in a single or multistagefluid bed, with air containing the chlorinating agent. The air is usedin such a quantity as to oxidize almost completely the magnetite tohematite. The gases flowing out of the reactor should not contain freeoxygen, which must be under 0.5 percent by volume. This can beaccomplished by analyzing the gases flowing out from the apparatus andproperly regulating the inlet of air and other gases. When the dew pointof the metal chlorides is greater than the operational temperature, itwill become necessary to dilute the air with inert gas, for instancewith an exhausted chlorination gas, previously dried. The chlorinatingagent is used in quantities of from -135 with respect to thestoichiometric quantity needed for converting the nonferrous metals intovolatile chlorides. The chlorinating agents are inorganic compounds suchas C1 BC], or chlorinated organic compounds, such as waste chloroalkanes(hexachloroethane, pentachloroethane, tetrachloroethane,chloropropanes). The chlorination is carried out between 650 and 1,000C., but preferably between 850 and 950 C., with contact times from 20 tominutes.

During the reducing phase, due to the high temperature, thereducing-chlorinating environment and the presence of water, thefollowing reactions take place:

Partial conversion of the hematite into magnetite;

sulfur removal (over 90 percent), the residual pyrite decomposes tomonosulfide, the sulfates for the most part are reduced to sulfides andto oxides (only small quantities of unreacted alkali earth sulfatesremain in the cinders), while the sulfides are hydrolyzed;

partial decomposition of the ferrites present (MO, F e 0 partialreduction of the oxides of Cu and Pb to metal Cu and Pb, when HCl ispresent in a sufficient quantity, the Pb volatilizes as chloride;

decomposition of the arsenates and the volatilization of the arsenicboth as As O and AsCl During the chlorination phase, the followingreactions take place:

the chlorination and volatization of the nonferrous metals;

The almost total reoxidation of the magnetite to hematite by the actionof the contained in the gas;

the further sulfur removal by the action of the reaction: M"SO, Cl M"ClSO 0 wherein M represents an alkali earth metal (calcium and barium),the reaction tends to shift to the right because the O is subtractedfrom the equilibrium by reacting with the hematite present; also lowcontents in 0 in the tail gases are quite sufficient to hinder thisreaction;

the further dearsenification by the action of the chlorinatingatmosphere which is free of O The main advantage, obtained by theprocess of this invention, is in the high dearsinification anddesulfuration yield that is attained during the purification from thenonferrous metals. The absence of the sulfur makes the pelletizing ofthe cinders possible at temperatures lower than those required for thedesulfuration and therefore allows the use of much cheaper techniques.Furthermore, the consumption of chlorination agent by the iron isreduced to a minimum. In the reduction stage, the water coming from thecombustion of the oil and present in a high percentage in the gases,hinders the chlorination of iron; the hydrolysis of the FeS during thereduction eliminates the main source of formation of the iron chloridein the successive chlorination phase.

Another advantage, offered by the process of this invention, concernsthe removal of the As and, in some cases, of the Pb, during thereduction phase. The aqueous solutions of the metal chlorides, obtainedby scrubbing the outlet gas from the chlorination are almost free ofsuch impurities which would mean trouble in the hydrometallurgicalprocess for the recovery of the nonferrous metals of greater value suchas Cu, Zn, Ag, Au, etc.

From the literature (German Pat. No. l,068,020) is already known aprocess by which one would obtain ashes purified from the nonferrousmetals and which will contain residual contents in As of from 0.03-0.05percent and in S of from 0.05-0.07 percent. The desulfuration, however,is accompanied by a heavy removal of Fe as chloride. The overall lossesin Fe are, in fact, of the order of 3.5-6 percent which correspond toconsumptions of chlorine of from 22 to 26 kg. per ton of cinders.Furthermore, the desulfuration cannot be complete, if the cinders to betreated contain alkali earth sulfates which are not decomposed under thecondition described in the cited German patent.

On the contrary, by operating according to the instant, invention,thanks to the previous elimination of S in the reduction phase, theoverall loss in Fe will be less than 0.7 percent and the correspondingconsumption of chlorine will be less than kg. per ton of cinders.Furthermore, according to the above-cited German Patent, thedearsenification occurs completely at the expenses of the chlorinatingagent, while with the process of this invention, thanks to the highertemperatures and to the atmosphere which is present in the reductionphase, the removal of arsenic occurs for the most part as A5 0 Thefollowing examples are given for illustrating the present inventionwithout however limiting the same. in these examples (as well as in thedescription preceding them and in the text following them) thepercentages and the parts are by weight, where nothing to the contraryis indicated. Nm means cubic meters reduced to normal conditions.

EXAMPLE NO. 1

From a roasting plant with fluid bed were discharged at a meantemperature of 500 C., 1,000 kg./hr. of Spanish pyrite cinders of thefollowing chemical composition (percent by weight):

Total Fe 61.50 total S 1.94 Monosull'tde S 0.93 As 0.33 Cu 0.88 Zn 2.50Pb 0.91 8:10 0.33 C210 0. l 3 Mg() 0.09 M 0 0.55 SiO, 3.45

These cinders were fed to a fluid bed reactor at the base of which wereinjected 27 kg./hr. of Bunker C fuel oil, 220 Nm. /hr. of air and 8kg./hr. ofa solution of HCl at 35.6 percent by weight. The hydrochloricacid corresponds to 60 percent of the stoichiometric for chlorinatingthe arsenic. The reactor operated at 900 C., the contact time of thesolids in the bed was 25 minutes. The reduced cinders showed thefollowing composition (in percent by weight):

Total S 0.16 Monosulfide S 0.05 As 0.07 Cu 0.9! Zn 2.57 Pb 0.89

These cinders were continuously fed hot into a one stage fluid bedreactor. The feeding to the bottom of the reactor consisted of 43 Nm./hr. of air, 59 NmP/hr. of exhausted and dried recycling gases (tappeddownstream of the scrubbing column of 50 chlorides), and 50 kgjhr. of C1The quantity of air fed was such as to ensure the practical absence of Oin the gases coming from the reactor (oxygen was 0.3 percent by volume).The quantity of CL used, diminished by the quantity of C1 for Ca and Ba,corresponded to l 15% of the theoretical quantity necessary forchlorinating the nonferrous metals (Cu, Zn, Pb).

The reactor operated at 950 C. with a contact time for the solids in thefluid bed of minutes. The discharged cinders showed the followingcomposition (in by weight):

Tote! Fe 66. K]

Total 5 0.0] 1 As 0.0l5 Cu 0.020 Zn 0.030 Pb 0.040

The losses in Fe through volatilization amount to 0.12 percent in thereduction phase and to 0.15 percent in the chlorination phase. Thecorresponding consumption of HCl and Cl is respectively 1.1 and 1.8kg./t. of treated cinders.

EXAMPLE NO. 2

1,000 kg./hr. of cinders at 500 C., described in the preceding example,were fed to a fluid bed reactor, at the base of which were introduced 26kg./hr. of Bunker C fuel oil, 215 NmF/hr. of air and 6.1 kgJhr. ofgaseous HCl (corresponding to about 80 percent of the stoichiometric forthe chlorination of lead and arsenic). The reactor operated at 900 C.,while the contact time of the solids was maintained at 35 minutes. Thecinders discharged from the fluid bed and from the cyclone showed thefollowing composition (in percent by weight):

Total Fe 64.20 Fe 1400 Total S 019 Monosulfide S 0.06 A5 0.042 Cu 0.91

These cinders feed a two stage fluid bed reactor. into the lower stageof the reactor were introduced: 40 Nmfi/hr. of air, 60 Nm. /hr. ofexhausted recycling gases (tapped downstream of the scrubbing column forthe chlorides and then dried), and 49.8 kg/hr. of HCl. The quantity ofair is such as to ensure the practical absence of O in the gases leavingthe reactor (oxygen 0.2 percent by volume), while the HC] used,diminished by the quantity of HCl for the Ca and the Ba, represents 120percent of the stoichiometric quantity required for chlorinating theresidual nonferrous metals. The reactor operated at 930 C. while theoverall contact time was 140 minutes. The discharged cinders showed thefollowing percentual composition:

Total Fe 66.00

Total S 0010 As 0.018 Cu 0.038 Zn 01045 Pb 0.030

The total loss in iron through volatilization was 0.64 percent, whilethe corresponding consumption of HCl was 5.15 kg. per ton ofashes.

We claim:

1. Process for the purification of pyrite and pyrrhotite cinders fromnonferrous metals, from arsenic and from sulfur, which comprisescarrying out the reduction of the hematite to magnetite in a fluid bed,by direct injection of a hydrocarbon fuel and air in deficiency, at850-950 C. and with contact time from 15 to 45 minutes, up to a degreeof reduction equal to from 10-90 percent, and in the presence of smallquantities of HCl, carrying out the chlorination and reoxidation of thethus produced hot cinders in a fluid bed reactor with air and achlorinating agent, at temperatures from 650-l ,000 C., with contacttime from 20 to 150 minutes, and that the gases leaving the reactorcontain less than 0.5 percent by volume of free oxygen.

2. The process of claim 1, wherein the hydrochloric acid in thereduction phase is used in a quantity which is from a minimum of 40percent of stoichiometric for chlorinating the arsenic, to a maximum ofpercent of stoichiometric for chlorinating As and Pb.

33. The process of claim 1, wherein the chlorinating agent is chlorineor hydrochloric acid.

4. The process of claim 1, wherein the chlorinating agent is a wastechloroalkane.

5. The process of claim 1, wherein the chlorinating agent is used in aquantity of from -135 percent with respect to the stoichiometricrequired for converting the nonferrous metals into volatile chlorides.

6. The process of claim 1, wherein the purified cinders are pelletizedat temperatures below 1,150" C.

7. The process of claim 1, wherein the purified cinders optionallyenriched, are sent hot directly to the partial or total reduction tometal iron.

8. The process of claim 1, wherein the chlorination and reoxidation isat a temperature of 850950 C.

2. The process of claim 1, wherein the hydrochloric acid in thereduction phase is used in a quantity which is from a minimum of 40percent of stoichiometric for chlorinating the arsenic, to a maximum of90 percent of stoichiometric for chlorinating As and Pb.
 3. The processof claim 1, wherein the chlorinating agent is chlorine or hydrochloricacid.
 4. The process of claim 1, wherein the chlorinating agent is awaste chloroalkane.
 5. The process of claim 1, wherein the chlorinatingagent is used in a quantity of from 105-135 percent with respect to thestoichiometric required for converting the nonferrous metals intovolatile chlorides.
 6. The process of claim 1, wherein the purifiedcinders are pelletized at temperatures below 1,150* C.
 7. The process ofclaim 1, wherein the purified cinders optionally enriched, are sent hotdirectly to the partial or total reduction to metal iron.
 8. The processof claim 1, wherein the chlorination and reoxidation is at a temperatureof 850*-950* C.